Yarn-driven idle or deflecting rollers are widely known in the synthetic fiber and textile industry. To treat this yarn as gently as possible, every attempt is made to reduce the driving force as low as possible. As the driving force is mainly necessitated because of friction, many attempts to reduce the driving force involves friction reducing devices for the rollers.
Air-cushioned designs have been especially suitable for this application because of the low friction performance characteristics of this type bearing. In particular, so-called aerodynamic bearing designs are suitable because of their ability to aspirate the air required for the formation of the "lubricating" film in operation.
A substantial drawback of the air bearing design, however, is the instability of the bearing during the starting phase of operation--i.e., from zero or low rotation to nominal rotational speed. Normally, contact between bearing elements is accepted during start-ups, and problems inherent therewith are overcome by selecting materials of construction having appropriate "self-lubricating" or friction minimizing properties. Even this technique, however, involves some frictional restraint during the start-up phase.
In manipulation of textile yarns, especially small denier yarns, the start-up phase is critical, as uneven and/or high tensions may break the yarn or cause it to wrap around the processing equipment. Aerostatic bearing designs--where high pressure air is continuously supplied--avoid the frictional problems during start-up, but use an unacceptable amount of energy and, therefore, cost.
An air-cushioned, aerodynamic idle roll which is started at high air pressure, and for which air at reduced pressure is used in the steady state condition, is known. See, for example, U.S. Pat. No. 3,746,233. FIG. 4 of that reference depicts two compressed air lines from a compressed air source to the idle roll. Each line has a pressure-reducing valve, one valve being adjusted to the pressure required during the initial start-up or acceleration phase of the roller, and the other line being adjusted for normal running conditions. Means for directing either of the two lines to the aerodynamic idle roll are also shown.